1. Field of the Invention
The field of the invention is that of the broadcasting of digital data designed to be received notably by moving receivers in an urban environment, namely in the presence of interferences or jamming, under conditions of multiple propagation (RAYLEIGH process) generating a phenomenon of fading.
The invention can be applied more particularly, but not exclusively, to a system of digital sound broadcasting as described in the French patent applications Nos. 86 09622 of Jul. 2, 1986 and 86 13271 of Sep. 23, 1986, on behalf of the same applicants. This system of digital broadcasting, presented in these prior patent applications, is based on the combined use of a channel coding device and a method known as the COFDM system (coding orthogonal frequency division multiplex system).
2. Description of the Prior Art
The modulation method proper of this prior art system consists in providing for the distribution of the constituent digital elements of the data signal in the frequency-time f-t space and in simultaneously emitting sets of digital elements on M parallel broadcasting channels by means of a multiplex of orthogonal carrier frequencies. This type of modulation makes it possible to prevent two successive elements of the data train from being emitted at the same frequency. This enables the absorption of the fluctuating selectivity in frequency of the channel, by frequentially dispersing the initially adjacent digital elements during the broadcasting.
The prior art encoding method seeks, for its part, to enable the processing of the samples coming from the demodulator to absorb the effect of variation in amplitude of the signal received, due to the RAYLEIGH process. This encoding is advantageously a convolutive encoding, possibly concatenated with a REED-SOLOMON type encoding.
In a known way, the encoded digital elements are furthermore interlaced, in time as well as frequency, in order to maximize the statistical independence of the samples with respect to the Rayleigh process and the selective character of the channel.